The next generation mobile communication and wireless communication systems require improved data transmission rates and system capacities in the multiple cell environments. In response to such demands, studies on a Multi-Input Multi-Output (MIMO) system transmitting data using a plurality of antennas have been conducted, and a closed loop type MIMO system, using channel state information to improve the data transmission rates in the multiple cell environments, enhances a transmission performance using the channel state information.
Generally, in the MIMO system, a mobile station can recognize information on a receiving channel using received data, whereas a base station cannot recognize the channel state information. Accordingly, the base station has to know the channel state information to improve a performance of the system using the channel state information.
The system using the closed loop MIMO transmits data using information on a transmitting channel of the mobile station serviced by the base station. At this time, the base station cannot recognize the information on the transmitting channel of the mobile station serviced thereby and therefore, the information on the channel, including a Channel Quality Indicator (CQI) and a Pre-coding Matrix Index (PMI), is fed back to the base station from the mobile station.
The mobile station estimates the channel through which data is received, using signals received from the base station. The mobile station calculates the CQI using the estimated channel in order to apply a Modulation Coding Scheme (MCS) suitable for the channel situation when the base station transmits data. Furthermore, channel coefficients most suitable for the channel situation, namely, a pre-coding vector of a code book is selected from the known code book by using the estimated channel and the code book. The channel state information which the mobile station obtains using the estimated channel is transmitted through a feedback channel between the base station and the mobile station. The base station transmits data to the mobile station using the channel information transmitted from the mobile station, the selected MCS, and the pre-coding vector of the code book.
A number of studies on the closed loop MIMO system have been currently conducted, and the closed loop MIMO is employed for the multiple cell environments as well as the single cell environment. However, more studies on the method for reducing effects by interference from other cells in the multiple cell environments are required.
In particular, a mobile station at an edge of a cell receives weak signals from a base station to which it belongs and thus, a base station of an adjacent cell receives interference due to a mobile station belonging to the adjacent cell. Accordingly, a performance of the mobile station located at the edge of the cell is degraded by such interference between the cells.
Furthermore, sub-cells configuring multiple cells in the multiple cell environments are allocated the same cell identifier (e.g., a Physical Cell ID (PCID)) irrespective of the number of transmitting antennas. If the cells are densely arranged on account of an increase in the amount of used data in such a situation, the interference between the cells is increased so that network geometry is decreased, and the performance is deteriorated due to overhead according to handover between the cells.